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Forces loyal to Libyan military rogue General Khalifa Haftar were pushed back on Friday from a key checkpoint less than 30 kilometres from Tripoli, checking their lightning advance on the capital, a security source said.

Militiamen from the coastal town of Zawiya, west of Tripoli, retook the base after a "short exchange of fire", the source said on condition of anonymity.

It comes after Haftar ordered his troops to "advance" on the capital Tripoli on Thursday, seat of the country's internationally-recognised unity government.

"The time has come. Tripoli we are coming," Haftar said in an audio message released online by his self-proclaimed Libyan National Army.

Haftar, who commands east-based LNA, described his forces' move as a "victorious march to shake the lands under the feet of the unjust."

He urged his forces to enter the city peacefully and only raise their weapons "in the face of those who seek injustice and prefer confrontation and fighting.

"Those who lay down their weapons are safe, and those who raise the white flag are safe," he added.

Unity government chief Fayez al-Sarraj had granted his air force permission to carry out strikes to repel "anyone who threatens the lives of civilians and vital facilities."

The Tripoli-based government had said that security forces in the capital were on "high alert" and condemned the threats of an armed showdown over the capital city.

"There is no military solution to this crisis as war only brings about destruction. All sides must stop using the rhetoric of escalation and replace it with wisdom," the statement said.

His interior ministry announced a "state of maximum alert" and powerful armed groups from Libya's western city of Misrata said they were ready to halt Haftar's advance.

A convoy of vehicles from Haftar's forces on Thursday pushed towards the city of Gharyan, some 100 kilometres (60 miles) from Tripoli, witnesses and military sources said.

Commander Abdessalem Al-Hassi said on Thursday that his forces had entered into the city without a fight.

But at least four sources in the city denied this, and a Gharyan official said there were "ongoing efforts to avoid a confrontation" between rival fighters who divide the city.

Haftar's LNA announced on Wednesday it was gearing up to move on the west of the country including the capital, home to the rival unity government.

His forces have emerged as a key player, opposing the government in Tripoli and backing a parallel administration in the east.

The rise in tensions came as UN boss Guterres visited Tripoli ahead of a planned conference later this month to hammer out a roadmap for delayed parliamentary and presidential elections.

"I am deeply concerned by the military movement taking place in Libya and the risk of confrontation," the visiting Guterres tweeted from the capital.

"There is no military solution" to Libya's woes, he added.

The militia is one of dozens that have proliferated since the overthrow of veteran dictator Muammar Gaddafi in a NATO-backed uprising in 2011 and are variously aligned with the UN-backed unity government in the capital and a rival administration in the east backed by Haftar's forces.